If you make the house too tight, your combustion appliances won't have enough air to burn right, and it can affect the flue. Then, the most effective and efficient thing to do is to use an air-to-air heat exchanger - it allows outside air to come into the house, but it uses a heat exchanger to recover some of the lost heat. Canada is ahead of us in mandatory energy efficiency items - this type of thing is required in new construction there.